Real-time prediction of transient stability after a fault can potentially prevent occurrence of grid blackouts. In this paper, the measurement data obtained from phasor measurement unit (PMU) located at generator buses are used to compute the transient stability margin after a fault has occurred. For evaluating various control actions to be taken to stabilize severely disturbed and unstable generators, the stability margin is estimated using the transient energy function (TEF) technique. A look-up table of various modes of disturbance (MOD) is built offline for different fault locations and post fault topology. Following an actual fault, the most probable MODs are ranked by matching the "normalized" kinetic energy with the look-up table. The correct MOD is then chosen based on the lowest normalized potential energy margin and the Controlling Unstable Equilibrium Point (CUEP) is calculated. This technique overcomes previously reported difficulties in finding the CUEP in real-time applications. With knowledge of prefault operating condition obtained from SCADA and the information of the tripped line by analyzing the PMU data, the first swing transient stability margin is computed. The amount of control action needed is subsequently calculated. The proposed method has been tested on the IEEE 39 Bus Test System.